Enhanced biosorption of wastewater organics using dissolved air flotation with solids recycle

Abstract

Systems and methods for treating wastewater including a dissolved air flotation operation performed upon a portion of a mixed liquor output from a contact tank prior to the mixed liquor entering a biological treatment tank.

What is claimed is: 
     
       1. A wastewater treatment system comprising:
 a contact tank having an first inlet configured to receive wastewater to be treated, a second inlet configured to receive activated sludge, and an outlet, the contact tank configured to mix the wastewater to be treated with the activated sludge to form a first mixed liquor; 
 a dissolved air flotation unit having an inlet in fluid communication with the outlet of the contact tank, the dissolved air flotation unit configured to separate suspended matter from a portion of the first mixed liquor to form a solids-lean dissolved air flotation unit effluent and waste solids, to output the waste solids through a solids outlet of the dissolved air flotation unit, and to output the solids-lean dissolved air flotation unit effluent through an effluent outlet of the dissolved air flotation unit; 
 a biological treatment unit having a first inlet in fluid communication with the effluent outlet of the dissolved air flotation unit, a second inlet, and an outlet, the biological treatment unit configured to biologically break down organic components of the effluent from the dissolved air flotation unit to form a second mixed liquor; 
 a clarifier having an inlet in fluid communication with the outlet of the biological treatment unit, an effluent outlet, and a return activated sludge outlet, the clarifier configured to output a clarified effluent through the effluent outlet and a return activated sludge though the return activated sludge outlet; 
 a dissolved air flotation unit effluent conduit extending between the effluent outlet of the dissolved air flotation unit and the first inlet of the biological treatment unit; and 
 a return activated sludge conduit extending between the return activated sludge outlet of the clarifier and the second inlet of the biological treatment unit. 
 
     
     
       2. The system of  claim 1 , wherein the biological treatment unit includes regions separated by a partition. 
     
     
       3. The system of  claim 1 , comprising a first sub-system including the contact tank and the dissolved air flotation unit which is physically separated from a second sub-system including the biological treatment unit. 
     
     
       4. The system of  claim 1 , including a plurality of dissolved air flotation units operating in parallel. 
     
     
       5. The system of  claim 1 , wherein the biological treatment unit includes an aerobic region. 
     
     
       6. A method of treating wastewater comprising:
 introducing the wastewater into a contact tank including an activated sludge; 
 mixing the wastewater with activated sludge in the contact tank to form a first mixed liquor; 
 directing the first mixed liquor to a dissolved air flotation unit; 
 separating the first mixed liquor in the dissolved air flotation unit to form a solids-lean dissolved air flotation unit effluent and waste biosolids; 
 directing the solids-lean dissolved air flotation unit effluent to a biological treatment unit; 
 biologically treating the solids-lean dissolved air flotation unit effluent in the biological treatment unit to form a biologically treated mixed liquor; 
 directing the biologically treated mixed liquor to a clarifier; 
 separating the biologically treated mixed liquor in the clarifier to form a clarified effluent and a return activated sludge; 
 recycling a portion of the return activated sludge to the biological treatment unit; and 
 directing the clarified effluent to a treated wastewater outlet. 
 
     
     
       7. The method of  claim 6 , wherein the dissolved air flotation unit removes between about 60% and about 100% of suspended solids in the first mixed liquor as waste biosolids. 
     
     
       8. The method of  claim 7 , wherein an amount of suspended solids removed in the dissolved air flotation unit is adjusted based upon a concentration of a bacteria in the biological treatment unit. 
     
     
       9. The method of  claim 6 , wherein the dissolved air flotation unit removes between about 40% and about 80% of biological oxygen demand in the first mixed liquor. 
     
     
       10. The method of  claim 6 , further comprising dosing the first mixed liquor with a coagulant. 
     
     
       11. The method of  claim 10 , wherein dosing the first mixed liquor with a coagulant includes dosing the first mixed liquor with ferric chloride. 
     
     
       12. The method of  claim 6 , wherein biologically treating the solids-lean dissolved air flotation unit effluent in the biological treatment unit includes aerobiacally treating the solids-lean dissolved air flotation unit effluent. 
     
     
       13. A method of facilitating increased operating efficiency of a wastewater treatment system comprising:
 providing a dissolved air flotation unit in a wastewater treatment system in fluid communication between a contact tank and a biological treatment unit, the dissolved air flotation unit configured to separate a portion of a first mixed liquor output from the contact tank to form a solids-lean dissolved air flotation unit effluent and waste solids, and to direct the solids-lean dissolved air flotation unit effluent into the biological treatment unit, the dissolved air flotation unit reducing the amount of biological oxygen demand to be treated in the biological treatment unit as compared to the wastewater treatment system operating in the absence of the dissolved air flotation unit; and 
 fluidly connecting a return activated sludge recycle conduit between a downstream solids-liquid separation unit and the biological treatment unit, the return activated sludge recycle conduit configured to recycle return activated sludge formed in the solids-liquid separation unit from a mixed liquor output from the biological treatment unit to the biological treatment unit. 
 
     
     
       14. The method of  claim 13 , further comprising reducing a rate of aeration of the biological treatment unit as compared to a rate of aeration of the biological treatment unit prior to providing the dissolved air flotation unit.